This invention relates to a novel method and apparatus for measuring the average size of a group of apertures in a member having an array of similar-sized apertures therein. The invention is especially applicable to measuring the apertures of an apertured mask for a cathode ray tube.
One type of color television picture tube employs a slit-type apertured mask. Such a mask is comprised of a metal sheet, about 4 to 8 mils thick (about 0.10 to 0.20 mm.), having an array of slits which are about 3 to 10 mils wide (about 0.075 to 0.25 mm.) whose center lines are substantially uniformly spaced about 20 to 80 mils (about 0.5 to 2.0 mm.) apart in parallel rows or columns. The slits may be of uniform width, or the widths may gradually become smaller from center to edge. The slits may be uniform, graduated, or random length in the rows, and are generally about 30 to 60 mils long. At least for quality-control purposes, it is desirable during manufacturing to check the widths of the slits from area-to-area on a single mask and also to check the widths of the slits from mask-to-mask.
It is known that projecting a beam of substantially monochromatic light from one side through an array of parallel slit apertures of substantially uniform widths and spacings produces combined interference and diffraction patterns in a plane spaced from the opposite side of the aperture array. The interference pattern is comprised of alternate light and dark bands of fringes. The diffraction pattern is an envelope defined by the peak intensities of the light fringes of the interference pattern. The envelope is also comprised of alternate light and dark bands including a central maximum and uniformly spaced side maxima (light bands) separated by minima (dark bands). The physical dimensions and spacings of the interference bands and diffraction bands are proportionately related to the average widths and spacings of the apertures which produce them. By prior methods, the average aperture width was derived by measuring one or more of these distances and then calculating the average width. Such prior methods are slow and the results are not as precise as is desired for quality control during mask manufacturing. Such prior methods do not lend themselves to automation by modern electronic techniques.